A series of earlier genetic studies on blood groups and isozymes indicated the existence of at least two separate subpopulations of skipjack tuna, Katsuwonus pelamis, in the Pacific Ocean and that the boundary between ranges of the different subpopulations shifts east-westerly by season in the offshore waters of the east coast of Japan and in the waters between the Bonin-Mariana Chain and the international date line in the northwestern Pacific Ocean. Insufficient data, however, had made such description difficult for the southwestern Pacific Ocean.More recent genetic studies on blood specimens from the southwestern Pacific Ocean indicate that the boundary between ranges of the above two subpopulations stays within the Tasman Sea all the year round and suggest that the western limit of the range of the central-eastern Pacific subpopulation(s) extends to the west close to the east coast of New South Wales in early winter in the southern hemisphere. On the basis of genetic data accumulated for the last eleven years, rejection limits of frequencies of an allele, E¹_sj, which determines the fastest band of serum esterase variants, were recalculated for the above two subpopulations.